Soft solder is a fusible alloy, typically consisting of tin and lead, which is used for the purpose of joining together two or more metals at temperatures below their melting points. In addition to tin and lead, solders may occasionally contain varying amounts of other materials, such as antimony, bismuth, cadmium, or silver, which are typically added for the purpose of varying the physical properties of the alloy. However, in many solders, some of these elements, notably antimony, are only present as impurities. Solder is widely used in the electronics industry for attaching electrical components to printed circuit boards. Printed circuit boards typically have an electrical conductor pattern consisting of a thin metal sheet, etched to form the pattern. In order to successfully attach the electrical components to the printed circuit board, there must be a metallurgical affinity between the two metals that are going to be soldered together, the metals must be free from all off-metallic surface contamination, there must be complete and adequate metallic contact between the solder and the metals that are to be soldered, and there must be a temperature adequate for sufficient alloying of the metal. The solder provides the attachment by virtue of an intermetallic solution, which takes place at the soldering temperature.
One method of assembling electronic assemblies is by a process known as solid solder deposition, where solder paste is printed and reflowed on the solderable surfaces of a printed circuit board (PCB) during the PCB fabrication. The advantage of this method is that the solder flux and the electronic components can then be placed directly on the PCB, without having to print solder paste. Two of the disadvantages of this method are: the paste that is clad onto the PCB results in a highly domed surface on the solder pads thereby degrading the accuracy of components placement; and the parts and flux tend to slide off of the domed surface when they are placed and while the circuit board is subjected to vibrations moving down the assembly line. Some have attempted to solve this problem by reducing the amount of solder paste dispensed onto the solder pad, thus decreasing the curvature of the domed surface. Others have attempted to flatten the domed solder surfaces by using a heated roller or platen. However, reducing the volume of solder paste decreases the total wetting forces of the solder on the pad and, therefore, reduces the tendency of the components to self center during the assembly reflow operation. This self-centering tendency is extremely important to assure accurate alignment and high-yield soldering. In addition, one would like to maximize the amount of solder deposited on the board during the cladding process in order to create a full solder fillet between the component and the PCB. Besides increasing soldering yields, solder joint reliability is negatively impacted when solder volumes are reduced.
Clearly, it would be a benefit to the electronics industry if a method of soldering could be devised that includes the advantages of both solid solder deposition and solder printing, but avoids the disadvantages of both processes.